X11 expertise on ancient HW sought... (4-plane visual (overlay) via X-server on MS-WIndows)

Mouse mouse at Rodents-Montreal.ORG
Sat Jun 6 07:35:07 CDT 2015

> I'm trying to get an application that currently uses a local display
> on an ancient DEC Alpha workstation with a (for the time)
> mid-to-high-end graphics controller (ZLX-E2) to instead use an
> X-server running under MS-Windows.

> The application is complaining that it cannot find a "4/5-bit
> visual".
> I conclude that the MS-Windows SW/HW system (X-server, MS-Win GPU
> driver, GPU) cannot offer 4-plane visuals.

Only for values of "cannot" that are more like "do not bother to
include the software to", I would say.

> I'm not even sure the Quadro 400 can handle 4bpp "visuals", or
> whatever MS-Windows calls them.

"Visual" is a technical term in X.  I don't know whether Windows has a
corresponding abstraction, so I don't know whether there is anything it
calls them.

> Am I correct to assume that the GPU must support 4bpp in order for it
> even to be possible for the X-server to propagate a 4-plane visual to
> a client?

No.  However, it is substantially more difficult for an X server to
present a visual that the hardware doesn't support, which is probably
why the server you have doesn't do it.  Well-behaved clients must be
prepared to handle whatever capabilities the server presents; the
problem here is that the client you have handles the server you have by
complaining and dying.

Depending on what other capabilities the client is using, you might be
able to get the overlay effect using colourmap hackery with
comparatively small code changes.  What PseudoColor or DirectColor
visuals are available?

> If yes, how can I determine if a GPU supports 4bpp?

Read the documentation, of course, or contact the manufacturer's
support department.

> Nvidia is very sparing with the information in their specs for the
> Quadro 400 GPU.

You may be out of luck without repalcing the hardware, then.  (That's
one of the prices of buying undocumented hardware....)

> Assuming I can find a GPU that supports/offers 4bpp, does anyone know
> an X-server product/project that can provide 4-plane overlays?

You probably do not need a GPU.  The era when 4-bit visuals were common
was full of dumb memory-mapped framebuffers; modern CPUs are fast
enough that they can probably fake up a 4-bit overlay visual and still
run at least as fast as the hardware your client software was designed
to run against.

I'm not sure how hard it would be to do.  My X server hackery has never
included faking something the hardware doesn't support, so my
experience is rather limited in that direction.  But I've done DDX
layers for at least three widely disparate framebuffers, and I feel
reasonably confident what you want could be done.

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